In general, the four basic elements of a process control loop include a process variable to be controlled, a process sensor or measure of the process variable's condition, a controller, and a control element. The sensor provides an indication of the process variable's condition to the controller, which also contains an indication of the desired process variable condition, or the "set point." The controller compares the process variable's condition to the set point and calculates a corrective signal, which it sends to the control element to exert an influence on the process to bring it to the set point condition. The control element is the last part of the loop, and the most common type of final control element is a valve, though it may also comprise a variable speed drive or a pump, for example.
A pressure regulator is a simple, self-contained control system that combines the process sensor, the controller and the valve into a single unit. Pressure regulators are widely used for pressure control in fluid distribution applications and the process industries, for example, to maintain a desired, reduced outlet pressure while providing the required fluid flow to satisfy a variable downstream demand. Pressure regulators fall generally into two main categories: direct-operated regulators and pilot-operated regulators.
A typical prior art direct-operated regulator 11 is illustrated in FIG. 1. Typical applications for direct-operated regulators include industrial, commercial, and gas service; instrument air or gas supply; fuel gas to burners; water pressure control; steam service; and tank blanketing. The direct-operated regulator 11 includes a regulator body 12 which has an inlet 13 and an outlet 14. A fluid flow passage area 15 having a restriction area 16 connects the inlet 13 and outlet 14. The restriction area 16 has a throttling element 17, such as a plug, membrane, vane, sleeve or similar restricting device which, when moved, limits the flow of the fluid (gas or liquid). An actuator including a sensing element having two sides responds to variations in the fluid pressure being controlled. Examples of sensing elements include membranes, diaphragms or pistons. The embodiment illustrated in FIG. 1 uses a diaphragm 18 for the sensing element. Control pressure is applied to the first side, or control side 19 of the sensing element via a control line or a passage 20 internal to the regulator body 12. If a control line is used for this purpose, it may be integral to the regulator body 12 or located in the adjacent piping. The second side, or reference side 21 of the sensing element is typically referenced to atmosphere. An additional force such as a spring 22 may be applied to the actuator, which biases the throttling element into a predetermined position representing a set point.
The direct-operated regulator 11 illustrated in FIG. 1 is considered a "pressure reducing" regulator because the sensing element (diaphragm 18) is connected by an internal passage 20 to pressure downstream of the regulator (on the fluid outlet-side) 14. An increase in downstream pressure is applied to control side 19 through the internal passage 20, applying pressure to the diaphragm 18, and forcing it up against the force of the spring 22. This, in turn, moves the throttling element up into the flow restriction area 16, reducing the fluid pressure to the regulator outlet 20.
Pressure reducing regulators regulate flow by sensing the pressure downstream of the regulator. A typical application of a pressure reducing regulators is on steam boilers, where pressure reducing regulators provide the initial pressure regulation. If the diaphragm 18 were connected to upstream pressure and the throttling element 17 were moved to the other side of the restrictor 16, the direct-operated regulator 11 would be considered a "back pressure" regulator. Back pressure regulators are applied, for example, in association with compressors to ensure that a vacuum condition does not reach the compressor.
A pilot-operated regulator is similar in construction to a direct-operated regulator. A typical prior art pressure reducing pilot operated regulator 23 is illustrated schematically in FIG. 2A, and a prior art back pressure pilot operated regulator is illustrated in FIG. 2B. The pilot operated regulator includes all the structural elements of the direct operated regulator with the addition of the pilot 24 (also called a relay, amplifier, or multiplier). The pilot is an auxiliary device which amplifies the loading pressure on the regulator actuator to regulate pressure. The pilot is similar in construction to a self operated regulator, having essentially the same elements as the self operated regulator.
In the pilot operated regulator 23 illustrated in FIG. 2A and FIG. 2B, inlet pressure is supplied via a pressure tap 27 in the piping upstream of the regulator 23. In the back pressure pilot operated regulator 23 in FIG. 2B, the pressure tap 27 further may include a restriction 26 therein. Inlet pressure to the pilot may also be supplied through an integral pressure tap to the regulator body. Outlet pressure is fed back through piping 20 connected downstream of the regulator 23. The downstream pressure is connected to the pilot 24 and the main regulator 10. The pilot 24 amplifies the pressure differential across the main regulator diaphragm 18 in order to control either the upstream (back pressure) or downstream (pressure reducing) fluid pressure.
Pressure regulators have many advantages over other control devices. Regulators are relatively inexpensive. They generally do not require an external power source to perform the pressure control function; rather, regulators use the pressure from the process being controlled for power. Further, the process sensor, controller and control valve are combined into a relatively small, self-contained package. Other advantages include good frequency response, good rangeability, small size, and there is generally little or no stem leakage.
There are also disadvantages associated with known regulators. Significant problems associated with existing pressure regulators include "droop" and "build-up," also referred to as offset or proportional band. Droop is defined as the decrease in controlled pressure in a pressure reducing regulator and build-up is defined as an increase in controlled pressure for a back pressure regulator that occur when moving from a low load to full load flow condition. They are normally expressed as a percent. Droop and build-up are especially prevalent with direct-operated regulators, but it also exists to a lesser degree with known pilot-operated regulators.
Regulators are often required to go to a no flow condition which is referred to as "lock-up" or "reseat." In a pressure reducing regulator such as the self operated regulator 11 in FIG. 1 or the pilot operated regulator 23 in FIG. 2A, down stream pressure may reach a point where it is desirable for the regulator 11 to completely stop fluid flow. At this down stream pressure, the control pressure fed back to the diaphragm 18 moves the throttling element 17 completely into the flow restriction area 16, thereby blocking flow. This condition is known as "lock-up." In a back pressure regulator such as the pilot operated regulator 23 shown in FIG. 2B, pressure up stream of the regulator may drop to a level where the regulator is required to shut off flow. In this case, the up stream control pressure falls to a level where the load spring and/or the pilot pressure cause the throttling element 17 to move to a position completely blocking fluid flow. Internal parts problems, contamination or binding in the movement of the internal parts can all contribute to a loss of lock-up capability.
Since a regulator is a self-contained control system, existing regulators typically do not contain the capability to communicate with other portions of a process control system. This creates several drawbacks. Since there is not a means to remotely provide a set point or tune a regulator, they generally must be adjusted manually. Adjustments are made by turning an adjustment knob on the regulator to achieve the desired force on the actuator. This is especially undesirable in remote applications or in processes controlling the pressure of hazardous substances. There are no control room indications of regulator performance, leaving operators to inferentially determine regulator malfunctions through readings of other process indications.
The lack of communications and processing capabilities may also lead to maintainability problems. It is difficult or impossible to closely monitor regulator performance over time, so there is little advance warning of the need to fix or replace a regulator. There is also a lack of advance warnings for impending failure, which is especially troublesome with existing pressure regulators: since they are process powered, they typically do not include a failure mode operation. If the operating diaphragm of a spring-loaded pressure reducing regulator fails, the regulator will open fully. This creates issues if the downstream piping cannot withstand upstream pressure conditions, or if a relief valve that can handle the maximum flow of the regulator is not present. Back-pressure regulators will completely close upon diaphragm failure, creating similar issues for the upstream portion of the process.
It is well known that in many situations to which pressure regulators could be applied, control valves are used instead. The control valve includes a powered actuator which is responsive to externally supplied signals for moving a throttling element to control flow. It has been estimated that properly utilized, regulators could replace control valves in 25% of applications using control valves. The hesitancy to use regulators in place of control valves is due, in large part, to the shortcomings associated with known pressure regulators. Primary concerns include droop characteristics and the lack of remote operability. Process equipment users, however, are continually looking to be more cost competitive. In addition to seeking improvements in process efficiency and up-time with existing process equipment, process equipment users are seeking lower cost solutions to process control. If the above discussed limitations of regulators were eliminated, they could provide a lower cost option for many control valve applications.
U.S. industries spend approximately $200 billion each year in maintenance of plant equipment. This results in maintenance costs representing 15-40% of the cost of goods sold per year. Further, one-third of the dollars spent on maintenance is wasted from unnecessary or ineffective maintenance. For example, since known regulators do not have diagnostics or communications capabilities to exchange information with external systems, they are difficult to troubleshoot. Often, in an attempt to correct unidentified process problems, regulators are replaced, only to learn that the regulator was functioning properly. Changing the regulator may require halting the entire process, resulting in significant lost production time. Improving the performance of process instruments such as pressure regulators as well as improving maintainability through processing capabilities and communications will significantly reduce manufacturing costs.
Thus, a need clearly exists for an improved pressure regulator that compensates for droop characteristics and exhibits improved performance. Further, it would be desirable for the improved regulator to include communication and diagnostic capabilities to allow remote operation and the exchange of data to enhance maintainability. Moreover, these additional features are required concurrently with the need for economical solutions for pressure regulation.